irqchip: nvic: Support hierarchy irq domain

Add support for hierarchy irq domains. This is required to stack
the MSCM interrupt router and the NVIC controller found in Vybrid
SoC.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Cc: marc.zyngier@arm.com
Cc: linux@arm.linux.org.uk
Cc: u.kleine-koenig@pengutronix.de
Cc: olof@lixom.net
Cc: arnd@arndb.de
Cc: daniel.lezcano@linaro.org
Cc: mark.rutland@arm.com
Cc: pawel.moll@arm.com
Cc: robh+dt@kernel.org
Cc: ijc+devicetree@hellion.org.uk
Cc: galak@codeaurora.org
Cc: mcoquelin.stm32@gmail.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: shawn.guo@linaro.org
Cc: kernel@pengutronix.de
Cc: jason@lakedaemon.net
Link: http://lkml.kernel.org/r/1431769465-26867-5-git-send-email-stefan@agner.ch


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>